Wireless LAN protocols
- Why wireless LAN?
o Convenience and flexibility. No need to carry wires
or retrofit buildings, (limited) mobility, no
connectors (e.g. Macbook Air).
- Why not wireless LAN?
o Not good if very high bandwidth is required
o RF compatibility and other safety issues
o Security (?)
IEEE 802.15.1 (Bluetooth)
- First let’s consider Bluetooth, a very simple wireless LAN
protocol – named for King Harald Bluetooth, a 10th
century Danish king
- Bluetooth connects small devices over a very short range
(~1-10m), depending on “class” – though longer ranges
are possible – “personal area network” for connecting
devices in your immediate vicinity
Bluetooth architecture
- Hierarchical “master-slave” system
- A collection of connected Bluetooth devices is a “piconet”
- Piconets consist of four kinds of device:
o Master (one per piconet)
o Slaves (up to seven per piconet, directly connected
to the master, actively transmitting data)
o Parked (known to the master but not transmitting
data)
o Standby (idle)
(Fig. 1)
- Any device can act as either master or slave; also,
piconets can overlap (so it’s maybe not appropriate to
think of the master as a “base station”)
- Active devices (master + slaves) are assigned a 3-bit
active member address (AMA) (this is why there can be at
most 7 slaves)
- Parked devices are assigned an 8-bit parked member
address (can be upgraded to active members as needed
or as a slot becomes available)
- It is possible for slaves to belong to more than one
piconet; it is also possible for a master to be a slave in
another piconet – however not simultaneously – the
devices jump back and forth
(Fig. 2)
- it is not possible for a device to be a master in two
piconets (in that case the piconets would merge into one)
Bluetooth Radio Interface
- Bluetooth operates in the 2.4GHz ISM band
- Multiple access via frequency hopping spread spectrum
(FHSS)
o 79 “hop carriers” each with 1 MHz of bandwidth
o 1600 hops/s (625 s per “slot”)
o so every 625 s, the system occupies a different,
pseudo-randomly selected 1 MHz frequency range
(Fig. 3)
- Why do this?
o Devices in the ISM band must tolerate interference
from other devices in the same band
o By randomly jumping from frequency to frequency,
it is unlikely (but not impossible!) that interference
will be encountered
- When a piconet is formed, the master sets the hopping
pattern
- All devices have a unique 48-bit device ID, which
establishes the hopping pattern (when master)
- Time-division duplexing: transmissions proceed as
master, slave 1, master, slave 2, master, etc.
- It is possible for transmissions to occupy 3 or 5 slots, as
needed – in this case no hopping is performed during the
block, and intermediate hops are skipped
(Fig. 4)
- data payload is up to 343 bits for single-slot packets
- What is the data rate? – given 1 slave, 2 slaves ... – for the
master
IEEE 802.11 (WiFi)
- The world’s most widely used wireless LAN standard
o 802.11 (1997): 1-2 Mbps
o 802.11a (1999): 27-54 Mbps (however, limited
range – only 802.11 outside of 2.4 GHz)
o 802.11b (1999): 11 Mbps
o 802.11g (2003): 22-54 Mbps
o 802.11n (2010?): 108-600 Mbps
- Compare with Ethernet – note – Cat 5 wire is most
commonly used with 100 Mbps and is temperamental at
higher data rates (e.g., 1 Gbps Ethernet) – with 802.11n,
all you need to do is upgrade the terminals